1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus, which generates a magnetic resonance image using the magnetic resonance phenomenon, and a method of judging presence or absence of unwanted mechanical resonance in this magnetic resonance imaging apparatus.
2. Description of the Related Art
A magnetic resonance imaging apparatus is an apparatus that visualizes chemical and physical microscopic information of a substance or observes chemical shift spectra utilizing a phenomenon in which, when a group of nuclei including peculiar magnetic moments are placed in a uniform static magnetic field, the group of nuclei resonantly absorb energy of a radiofrequency magnetic field rotating at a specific frequency.
Imaging of a diagnostic image by this magnetic resonance imaging apparatus is executed, for example, as described below. A patient is arranged in a composite magnetic field that consists of a static magnetic field formed by a magnet and a gradient magnetic field formed by a gradient magnetic field coil. A predetermined Larmor RF frequency for generating the magnetic resonance phenomenon is applied to the patient set in this way. A magnetic resonance signal is generated in the patient by the applied RF signal. The magnetic resonance imaging apparatus receives this magnetic resonance signal with a radiofrequency coil for reception and visualizes the magnetic resonance signal.
In the magnetic resonance imaging apparatus, since the gradient magnetic field changes within the static magnetic field while collecting data, a dynamic load due to the Lorentz force is applied to the gradient magnetic field coil structures. A dynamic load is also applied to the static magnetic field structures because of electromagnetic induction or propagation due to mechanical combination with the gradient magnetic field coil. When a frequency of this imposed mechanical load approaches a natural resonant frequency of a support system of the gradient magnetic field or a natural resonant frequency of the magnet, wherein resonance is caused. When resonance starts to occur, the amplitude of mechanical motion oscillation increase, which causes image blur or deterioration in image quality due to signal deterioration in a sequence for forming an image using plural echoes such as an FSE (Fast Spin Echo) method. Other than the influence on image quality, there are also deficiencies such as an unpleasant feeling given to a patient.
Note that a technique for reducing noise caused following a change in a gradient magnetic field waveform is known through, for example, JP-A-4-208134. In JP-A-4-208134, intensity and application time of a read pulse are set in predetermined ranges, a spectrum of the set read pulse is estimated, and power of noise caused by the read pulse is estimated from a measured noise characteristic. There is disclosed a technique for controlling noise in a sequence for switching a read pulse at high speed such as EPI (Echo Planar Imaging) by estimating a read pulse at which the estimated power of noise is minimized. According to the disclosure of JP-A-4-208134, since a response function spectrum of noise has plural peaks (modes of resonance), it is possible to reduce a noise level by shifting a basic frequency or the like of the read pulse sequence from the peaks appropriately.
However, in JP-A-4-208134, since attention is only paid to spectrum for one slice period, the measured noise concerns only oscillation of a relatively high frequency. Thus, it is likely that prior noise controlling effects are insufficient to avoid other imaging problems as noted above.
As described above, conventionally, resonance of a natural oscillation following a dynamic load occurs to deteriorate image quality and cause deficiencies such as an unpleasant feeling given to a patient.